1. Field of the Invention
The invention relates to a method of titanium nitride film deposition and, more particularly, to a method of forming a thick, crack-free titanium nitride film.
2. Description of the Background Art
In the manufacture of integrated circuits, a titanium nitride film is often used as a metal barrier layer to inhibit the diffusion of metals into an underlying region beneath the barrier layer. These underlying regions include transistor gates, capacitor dielectric, semiconductor substrates, metal lines, and many other structures that appear in integrated circuits.
For example, when an electrode is being formed for a transistor""s gate, a diffusion barrier is often formed between the gate material and a metal that serves as the contact portion of the electrode. The diffusion barrier inhibits the diffusion of the metal into the gate material, which may be composed of polysilicon. Such metal diffusion is undesirable because it would change the characteristics of the transistor, or render it inoperative. A combination of titanium/titanium nitride (Ti/TiN), for example, is often used as a diffusion barrier.
The Ti/TiN stack has also been used to provide contacts to the source and drain of a transistor. For example, in forming a contact using a tungsten (W) plug process, a Ti layer is deposited upon a silicon (Si) substrate, followed by conversion of the Ti layer into titanium silicide (TiSix), which provides a lower resistance contact with Si. A TiN layer is then formed upon the TiSix layer, prior to forming the tungsten plug. In addition to being a barrier layer, the TiN layer serves two additional functions: 1) prevents chemical attack of TiSix by tungsten hexafluoride (WF6) during W deposition; and 2) acts as a glue layer to promote adhesion of the W plug.
Ti and TiN films can be formed by physical or chemical vapor deposition. A Ti/TiN combination barrier layer may be formed in a multiple chamber xe2x80x9ccluster toolxe2x80x9d by depositing a Ti film in one chamber followed by TiN film deposition in another chamber. When depositing both Ti and TiN using chemical vapor deposition (CVD), titanium tetrachloride (TiCl4), for example, may be used to form both Ti and TiN films when allowed to react with different reactant gases, i.e., under plasma conditions, Ti is formed when TiCl4 reacts with H2, and TiN is formed when TiCl4 reacts with nitrogen. In general, TiN can be formed by reacting TiCl4 with a nitrogen-containing compound under either plasma or thermal conditions, depending on the specific nitrogen-containing compound. Thus, a TiN film may be formed by high temperature CVD using a reaction between TiCl4 and ammonia (NH3). However, such a TiN film tends to have intrinsically high tensile stress, e.g., on the order of 2xc3x971010 dyne/cm2 for a film thickness of 200 xc3x85. Since tensile force increases with increasing film thickness, cracks begin to develop as the thickness exceeds about 400 xc3x85. In fact, both the density and size of the cracks increase with film thickness, until the film eventually peels off.
Therefore, a need exists in the art for a method of forming reliable thick TiN films having improved properties such as good step coverage and low stress.
The present invention is a method of forming a titanium nitride (TiN) layer using a reaction between NH3 and TiCl4. In one embodiment of the invention, a TiN layer is formed at a temperature of less than about 550xc2x0 C. and a pressure between about 10-50 torr. More preferably, the TiN layer is formed at a temperature of about 500xc2x0 C., a pressure of about 20 torr and an NH3:TiCl4 ratio of about 8.5.
In another embodiment, a TiN layer is formed by depositing alternate TiN layers using two process steps having different NH3:TiCl4 ratios. Preferably, both process steps are performed at a temperature of about 500xc2x0 C. The alternate TiN layers differ in their film characteristics, such as stress, step coverage and crystal structure. A final TiN layer comprising a composite of these alternate layers has an improved overall step coverage and stress properties compared to a TiN layer deposited using prior art processes. In one preferred embodiment, a first TiN layer is deposited to a first thickness, for example, less than about 20 xc3x85, at an NH3:TiCl4 ratio between 40 and 250. This first TiN layer tends to have a lower film stress. This is followed by depositing a second TiN layer to a second thickness, for example, between 150-300 xc3x85, using a NH3:TiCl4 ratio between 2.5 and 17, or preferably about 8.5. The second TiN layer has excellent step coverage but higher film stress. By repeatedly forming these two alternate layers, a final composite TiN layer, e.g., over 1000 xc3x85 thick, can be formed with an overall improvement in both step coverage and film stress. In general, the specific process step used to form the initial TiN layer is immaterial, and the alternate layers can be deposited to different thicknesses as appropriate. The composite TiN layer formed using the present invention is well-suited for plug-fill applications for geometries at or below 0.18 xcexcm.